Structural porous metal (SPM) parts comprising a porous metal core and metal face sheets bonded to opposite sides of the porous metal core have been proposed for use in various applications. For example, U.S. Pat. No. 5,564,064 to Martin describes producing integral metal structures having porous cores for various applications. In Martin, a shell container is initially prepared from a solid metal material, filled with a reactively compatible metal core material and a gas, and sealed. The sealed, gas-filled shell container is heated and hot isostatically pressed to produce a consolidated metal billet wherein the gas is trapped in the metal core. The consolidated billet is then rolled, sectioned and welded together to produce shaped billets having a predetermined geometry. In addition, the billet is often machined to provide predetermined thicknesses across the SPM structure.
The billet is then annealed to force the gas out of solid solution and expand the structure to produce a SPM structure in situ.
Although SPM structures having predetermined geometries can be produced according to the method described in Martin, there are problems associated with this method. In particular, by sectioning and welding together different billets, the material strength and stiffness properties of the SPM structure is decreased. In addition, discontinuities in the shaped SPM structure generally exist at welding points. Furthermore, machining the face sheets of these structures reduces the benefits of forming an in situ sandwich structure.
The negative effects of welding structural pieces together and machining the face sheets to form the desired structure shape are especially noticeable when fasteners are applied to these structures. In particular, any discontinuity at the surface of the structure concentrates stress and provides a point of crack initiation and ensuing failure of the components. In addition, if fasteners are applied in the area of porous expansion, fatigue cracks can initiate at the pores exposed by machining as well as at the interface between the face sheets and the porous metal core and grow at an accelerated rate to failure.
One solution to this problem has been to use some method of cold-working around the drilled holes or thru-machined areas to densify the porous metal core at the edges and increase the fatigue life of the component. Nevertheless, this additional densification step is time-consuming and increases manufacturing cost.